A number of air driven percussion drilling tools are in use today which embody the hammer and anvil principle of converting fluid energy to mechanical energy for drilling purposes and are the accepted standard of the state of the art. Very few if any significant improvements have been made in recent years to advance the state of the art, so that all known equipment suppliers provide somewhat similar hammers and bits with minor differences. Examples of these tools are disclosed in U.S. Pat. No. 4,054,180 to Bassinger; U.S. Pat. No. 3,712,388 to Curington; U.S. Pat. No. 4,015,670 to Rear; and U.S. Pat. No. 3,768,576 by Martini.
The above-listed tools are comparatively large, have high stress levels at percussion, and have comparatively low cycle frequency. Because the mandrel-bit is massive, in order to achieve reasonable energy transfer from the piston, it must also be comparatively massive resulting in slower piston accelerations and tool cycle frequency for a given fluid energy supply. Also, the pistons are usually of less weight than the mandrel-bit resulting in instantaneous piston reversals at percussion leading to shock fatigue stresses and energy retension in the piston with wasted fluid pressure on the piston upstroke.
Also, when the cutting structure of the mandrel-bit becomes worn underside on the diametrical hole gage row of inserts or excessive flats are worn on the cutting tips of the inserts; or if the fixed non-replacable pressed in position inserts are broken, the whole large extendable splined mandrel-bit must be replaced as a unit incurring expensive replacement costs.
Other disadvantages include high drawdown forces that are required to keep the tool case closed with the mandrel-bit during operation, and tool length which may be a problem on some drilling rigs since the tool is installed under the rig rotary table. Another disadvantage on some drills is that no provision is made for air volume adjustment to provide suitable annulus air velocity for different bit sizes, while other drills require major disassembly and reassembly of the drill to make the necessary air volume adjustments.